Graphene-Derived Carbon Support Boosts Proton Exchange Membrane Fuel Cell Catalyst Stability

The lack of efficient and durable proton exchange membrane fuel cell electrocatalysts for the oxygen reduction reaction is still restraining the present hydrogen technology. Graphene-based carbon materials have emerged as a potential solution to replace the existing carbon black (CB) supports; however, their potential was never fully exploited as a commercial solution because of their more demanding properties. Here, a unique and industrially scalable synthesis of platinum-based electrocatalysts on graphene derivative (GD) supports is presented. With an innovative approach, highly homogeneous as well as high metal loaded platinum-alloy (up to 60 wt %) intermetallic catalysts on GDs are achieved. Accelerated degradation tests show enhanced durability when compared to the CB-supported analogues including the commercial benchmark. Additionally, in combination with X-ray photoelectron spectroscopy Auger characterization and Raman spectroscopy, a clear connection between the sp2 content and structural defects in carbon materials with the catalyst durability is observed. Advanced gas diffusion electrode results show that the GD-supported catalysts exhibit excellent mass activities and possess the properties necessary to reach high currents if utilized correctly. We show record-high peak power densities in comparison to the prior best literature on platinum-based GD-supported materials which is promising information for future application.


Pulse combustion reactor synthesis
The exact procedure is already published elsewhere. 2 In the case of the synthesis of carbon black supported M/C composite materials, the feedstock suspension consisted o f suspending 30 g carbon black (Ketjen Black EC300J) and a metal acetate hydrate in 1500 mL ultrapure water (resistivity 18.2 MΩ cm, obtained from Milli-Q Direct Water Purification System, MilliPore). In the case of Co/C composite, 75.4 g of cobalt acetate tetrahydrate (Sigma Aldrich) was used and in case of Cu/C composite, 60.5 g of copper acetate monohydrate (Sigma Aldrich). For the synthesis of Co/rGO composite, the feedstock suspension consisted of dissolving 25.0 g of cobalt acetate monohydrate (Sigma Aldrich) in 1 L of ~14 g GO L -1 ultrapure water suspension, and in the case of Cu/rGO composite, the feedstock suspension consisted of dissolving 21.0 g of copper acetate monohydrate (Sigma Aldrich) in 1 L of ~14 g GO L -1 ultrapure water suspension. Lastly, all feedstock suspensions were mixed vigorously using a homogenizer (Ultra-turrax T-25 basic, IKA) for 10 minutes before being introduced to the PC reactor. Each feedstock suspension was then continuously stirred with a mechanic stirrer (IKA) while being fed into the reactor using a peristaltic pump. Specific details regarding the working principle of pulse combustion reactor have already been published elsewhere. [3][4][5] During the synthesis, supported metal nanoparticles (SMNPs) are collected with the electrostatic filter, mounted behind the reaction pipe.

Synthesis of intermetallic Pt-alloy electrocatalysts
Both the CB and GD supported intermetallic Pt-alloy analogues were prepared in accordance with the processes already reported previously. [6] Briefly, the electrocatalysts have been prepared in two steps. In the first step, Pt NPs were deposited onto the M/C or M/GD composites by using a part of the base metal M as sacrificial via the double passivation galvanic displacement method reported. 6 In the second step, the prepared composites with deposited Pt NPs were thermally annealed in order to obtain an intermetallic crystal phase. 7 For the purpose of GDE evaluation, all the evaluated catalysts were also acid washed (de-alloyed) in accordance to the work described previously. 8,9

ICP-OES and digestion
All reagents used were of analytical grade or better. For sample dilution and preparation of standards, ultrapure water (resistivity 18.2 MΩ cm, obtained from Milli-Q Direct Water Purification System, MilliPore) and ultrapure acids (HNO 3 and HCl, Merck-Suprapur) were used. S4 Standards were prepared in-house by dilution of certified, traceable, inductively coupled plasma (ICP)-grade single-element standards (Merck CertiPUR). A Varian 715-ES ICP optical emission spectrometer (OES) was used. Before ICP-OES analysis, each electrocatalyst was weighted (approximately 10 mg) and digested using a microwave-assisted digestion system (Milestone, Ethos 1) in a solution of 6 mL HCl and 2 mL HNO 3 . Samples were then filtered, and the filter paper was again submitted to the same digestion protocol. These two times digested samples were cooled to room temperature and then diluted with 2 V% HNO 3 until the concentration as required.

XRD analysis
The powder X-ray diffraction (XRD) measurements of samples containing Ni and Cu were carried out on a PANalytical X'Pert PRO MPD diffractometer with Cu Kα1 radiation (λ = 1.5406 Å) in the 2θ range from 10° to 60° with the 0.034 o step per 100 s using full opened X'Celerator detector.
Samples were prepared on zero-background Si holder.
The powder X-ray diffraction (XRD) measurements of samples containing Co were carried out on a PANalytical X'Pert PRO diffractometer with Cu Kα radiation (λ = 1.541874 Å) in the 2 θ range from 10° to 60° with the 0.039 o step per 300 s using full opened Pixel detector. Samples were prepared on zero-background Si holder.

Transmission Electron Microscopy (TEM) and scanning TEM (STEM) analysis
TEM and STEM imaging was carried out in a probe Cs-corrected scanning transmission electron microscope Jeol ARM 200 CF operated at 80 kV. Different regions of the samples were inspected in order to gather information from the most representative parts. For STEM an alysis powder samples were transferred to lacey-carbon coated copper or nickel grids. TEM images were used to do the particle size distribution using ImageJ software.

Raman characterization
The Raman spectra were recorded in the spectral range from 50 to 3700 cm -1 using an Alpha 300 confocal Raman spectrometer (WITec, Ulm, Germany) with 20x or 50x objective. Green laser with an excitation wavelength of 532 nm was used with a laser power ranging from 0.1 to 1 mW.
The spectra were recorded up to 100 scans and integration times from 1 to 20 s depending on the sample. For each sample, three different locations were analyzed to verify the spectra.

Scanning electron microscope (SEM) and energy dispersive X-ray (EDX) analysis
Scanning electron microscope (SEM) analysis was performed on SUPRA 35 VP (Carl Zeiss) microscope at 5 kV using In-lens detector. Standard SEM pin mounts (Agar scientific) covered with conductive carbon tape (Agar scientific) were used to hold the powder samples.
Energy dispersive X-ray (EDX) analysis was performed using detector SDD Ultim max 100 (Oxford, UK) at 5 kV for PtCu samples and 20 kV for PtCo samples. Samples were prepared using the following procedure: Small amount of powder electrocatalyst sample (1 -3 mg) was put on 13 mm polished metal disk and covered with the metal disk of the same size. Samples were pelleted with the manual press until the pellet with a thickness of about 50 µm was obtained. Standard SEM pin mounts (Agar scientific) covered with conductive carbon tape (Agar scientific) were used to hold pelleted samples.

X-ray photoelectron spectroscopy (XPS) analysis
X-ray photoelectron spectroscopy (XPS) was performed with the AXIS supra+ instrument (Kratos, Manchester, UK) using a monochromatic Al K α X-ray source. The powder samples were immobilized on conductive carbon tape (Agar scientific) attached to a Si wafer (Agar scientific).
The powder samples completely covered the entire surface of the carbon tape. The samples prepared in this way were fixed on the sample holder with conductive carbon tape. For each sample, spectra were acquired on a 300 by 700 µm analysis spot size. Survey spectra were measured at pass energy of 160 eV and an emission current of 15 mA, while high -resolution (HR) spectra were measured at pass energy of 20 eV and an emission current of 15 mA. For the measurements of the HR Auger C KLL spectra, at least 15 sweeps were performed to improve the S/N ratio as these spectra are located at relatively high binging energy.

Electrochemical evaluation via Thin Film Rotating Disc Electrode (TF-RDE)
Preparation of thin films and the setup -Electrochemical measurements were conducted with a CompactStat (Ivium Technologies) in a two-compartment electrochemical cell in a 0.1 M HClO 4 (Merck, Suprapur, 70 wt%, diluted by ultrapure water (18.2 MΩ cm)) electrolyte with a conventional three-electrode system ( Figure S16a). Hydrogen electrode (Gaskatel) was used as a reference and a graphite rod as a counter electrode. The working electrode was a glassy carbon disc embedded in Teflon (Pine Instruments) with a geometric surface area of 0.196 cm 2 . Prior to S6 each experiment, the two-compartment electrochemical cell was boiled in ultrapure water for 1 hour, and the electrode was polished to mirror finish with Al 2 O 3 paste (particle size 0.05 µm, Buehler) on a polishing cloth (Buehler). After polishing, the electrodes were rinsed and ultrasonicated (Iskra Sonis 4, Iskra) in ultrapure water/isopropanol mixture for 3 min. 20 µL of 1 mg mL -1 water based, well dispersed electrocatalyst ink was pipetted on the glassy carbon electrode completely covering it and dried under ambient conditions. After the drop had dried, 5 µL of Nafion solution (ElectroChem, 5 wt% aqueous solution) diluted in isopropanol (1:50) was added. The electrode was then mounted on the rotator (Pine Instruments).
For catalysts not chemically activated, an electrochemical activation protocol was used before proceeding to the activity measurements. The working electrode was rotated at 600 rpm in Ar saturated electrolyte, 200 cycles in the potential window 0.05-1.2 V RHE were conducted at a scan rate of 300 mV s -1 . After the activation step, the electrolyte in the cell was exchanged for the fresh one before proceeding with the next step.
For all electrocatalysts, the electrodes with electrocatalyst films were placed in the oxygen saturated electrolyte without any potential control (at the OCP) and ORR polarization curves were measured at 1600 rpm in the potential window 0.05-1.0 V RHE with a scan rate of 20 mV s -1 immediately after measurement of ohmic resistance of the electrolyte (determined and compensated for as previously reported). 10 At the end of ORR polarization curve measurement, the electrolyte was purged with CO under potentiostatic mode (0.05 V RHE ) in order to ensure successful CO adsorption. Afterward the electrolyte was saturated with Ar. CO-electrooxidation was performed using the same potential window and scan rate as in ORR, but without rotation and in an Ar saturated electrolyte. After subtraction of background current due to capacitive currents, kinetic parameters were calculated at 0.95 V RHE by using Koutecky-Levich equation. 11 Electrochemically active surface area (ECSA CO ) was determined by integrating the charge in COelectrooxidation ('stripping') experiments as described in the reference. 12 All potentials are given against the reversible hydrogen electrode (RHE).

High temperature (HT) stability evaluation via accelerated degradation test (ADT) using HT Disc Electrode (HT-DE) setup
The accelerated degradation tests (ADTs) were performed in a two -compartment cell in 0.1 M HClO 4 (Merck, Suprapur, 70 wt%, diluted by ultrapure water (18.2 MΩ cm)) electrolyte with a conventional three-electrode system controlled by a potentiostat CompactStat (Ivium S7 Technologies). A hydrogen electrode (Gaskatel) was used as a reference electrode (RE), and a graphite rod (Sigma Aldrich) was used as a counter electrode (CE), The preparation of the setup and the thin films, was the same as described above. The in-house setup used is presented in Figure   S16b.
The electrochemical potential cycling activation and ORR polarization curve and COelectrooxidation measurements took place within the standard TF-RDE measurement setup at RT (in accordance with the same procedures described in the previous chapter). After electrochemical evaluation prior to any ADTs, the disc electrode was transferred to the HT-DE setup where 5000 cycles (0.4-1.2 V RHE ) with scan rate of 1 Vs -1 at 60 °C were performed. After the ADT, the disc electrode was transferred back to the standard TF-RDE setup and ORR polarization curve as well as CO electrooxidation were measured once again (also at RT). Stability was evaluated based on the CO-stripping performance before and after ADT.

Electrochemical evaluation with Gas Diffusion Electrode (GDE)
Electrode manufacturing -GDE manufacturing was done following the protocol used in previous work. 13 The ink for the GDE fabrication comprised of a total 1 wt% solids in a solvent mixture of  Figure S1 shows a pulse combustion reactor synthesis reproduced in a batch mode inside of tube furnace. GO suspension with added adequate amount of metal acetate (M+GO/GONR Figure S1a) is first put in i) ceramic crucible as is-in liquid form (Figure S1b) or ii) freeze dried to avoid the (1.) synthesis step that involves solvent evaporation (Figure S1c). The crucible is put into a quartz glass tube in Ar atmosphere to avoid an oxidative environment. The crucible with the glass tube is then inserted in the tube furnace to start the synthesis. However, slower reaction conditions of a batch system involving a slower heating ramp of 10 K min -1 prevent the fast reaction rate that is possible to achieve in the PC reactor. Therefore, this synthesis step is prolonged to a much longer time measured in hours instead of seconds as with PC reactor. Also, the conditions in the crucible are far less ideal in comparison to PC reactor leading to a more inhomogeneous product presented in the SEM image ( Figure S1d). Even from SEM image, it can be observed, that the particle size distribution is much broader, containing several size fractions of different nanoparticles, with the largest measuring almost 400 nm in diameter, which is too large for application in electrocatalysis. 15 S10 Figure S2. ADF and BF STEM imaging of PtCu/rGO electrocatalyst.         Table S1 shows the weight percentages of metal content derived from the EDX analysis. Here, the synthesized samples were characterized prior to the ex-situ chemical activation (de-alloying) of the samples described in the experimental section. This explains the high content of M, since the majority is removed during the activation, where also platinum rich over layer is formed. In Table S2 presents the weight percentages of metal content analyzed using ICP-OES. Here the chemically activated (de-alloyed) samples were characterized, which explains the lower values of M. These were also the values that were used in the recipe for catalyst ink preparation 13    pronounced. This may be since the as-received Umicore sample is already in active form (dealloyed), compared to the synthetically produced PtCo analogues (non-dealloyed), which still have a higher metal content. This also agrees well with the EDX analysis in Table S1 and the ICP-OES data in Table S2.  This is also an indication that the CB material did not chemically change during the synthesis process. To further check the sp 2 content in each sample, deconvolution of the high-resolution C 1s spectra was performed ( Figure S17). A Shirley background was used in all cases, with the S22 maximum of the C 1s spectra fixed at a binding energy of 284.5 eV (due to higher sp 2 content than sp 3 ). The full width at half maxima (FWHM) of all deconvoluted peaks was kept the same for fitting. In the fitting procedure, the ratio of sp 2 and sp 3 content as determined by the D-parameter method was fixed for all samples ( Table S3). The fitted curve agrees well with the measured data, indicating the adequacy of the procedure used. Figure S16. The Auger C KLL spectra processing; (a) measured Auger C KLL spectra, (b) smoothed Auger C KLL spectra using SG quadratic (9) function, (c) 1 st derivative of C KLL Auger spectra using SG quadratic (5) function, (d) smoothed Auger C KLL spectra using SG quartic (15) function, (e) 1 st derivative of Auger C KLL Auger spectra using SG quadratic (5) function.   Figure S18. Comparison of (a) conventional, two-compartment TF-RDE setup and (b) in-house designed HT-DE setup for performing ADTs at elevated temperatures using a thermostat. 24 Figure S18a presents the scheme of a conventional, two compartment TF-RDE setup which was used for ECSA CO and MA determination. Figure S18b presents a scheme of an in-house designed HT-DE setup which was used to perform ADTs. In this case the e lectrolyte evaporation is mitigated via a refluxing column. The electrocatalyst evaluation such as electrochemical activation (in situ EA) as well as ORR polarization curve or CO-electrooxidation/HUPD CV measurements take place in the TF-RDE setup at RT, while disc electrode is transferred to the HT-DE setup for ADT at elevated temperatures (e.g., 60 °C). Performance after successful ADT is re-evaluated in the normal two compartment TF-RDE setup ( Figure S18a). ECSA CO values after EA (before ADT) is shown in Figure S19a, whereas ECSA CO values after ADT are presented in Figure S19b.

Rotating disc electrode results
Examples of CO-stripping measurements from which ECSA CO data was derived from are presented in Figure S20.